It is known that a conventional spark plug has a structure provided with a center electrode, an insulator for holding the center electrode and a main metal fitting which is equipped with a ground electrode at its tip end portion and has a tool engagement portion for mounting on an engine, and the insulator is supported and fixed in the main metal fitting. Such a spark plug generally has a structure that the insulator is inserted into the main metal fitting having a cylindrical shape, and one end of the main metal fitting is caulked to support and fix the insulator in it (see, for example, Patent Reference 1).
The insulator is cylindrical and has a large-diameter portion which is formed on the intermediate potion in the axial direction of the insulator to protrude in a radial direction to have a flange shape and a largest outer diameter, an intermediate-diameter portion which has an outer diameter smaller than the large-diameter portion formed adjacent to the tip end side of the large-diameter portion, and a small-diameter portion which is formed on the tip end side of the intermediate-diameter portion via a step portion having an end surface facing the tip end and has an outer diameter smaller than the intermediate-diameter portion. Meanwhile, a rear end-side body portion which has an outer diameter smaller than the large-diameter portion and keeps a substantially constant outer diameter up to the rear end of the insulator is formed on a rear end side of the large-diameter portion. The center electrode is disposed at the tip end side on the inside of the insulator, and a metallic connecting terminal is connected to it via a conductive glass seal or a resistor. The connecting terminal is disposed to partly protrude from the rear end of the insulator.
A general spark plug such as the one of Patent Reference 1 having the above-described insulator has the rear end portion of the main metal fitting caulked inward in the radial direction to enable to push the large-diameter portion of the insulator directly or indirectly via talc or the like toward the tip end in the axial direction, so that the step portion of the insulator is pushed to the engagement portion which is protruded inward in the radial direction of the main metal fitting. The step portion and the engagement portion are engaged directly or indirectly with an intervening substance such as a packing or the like therebetween to maintain airtightness between the insulator and the main metal fitting. Thus, to push the insulator from the main metal fitting toward the tip end in the axial direction, it is necessary to form a flange-shaped large-diameter portion on the insulator.
However, the formation of the large-diameter portion as described above prevents the spark plug from having a smaller diameter. Therefore, it cannot fully meet the demand from the engine side that the spark plug is desired to have a smaller diameter. Accordingly, there is also proposed a spark plug having the insulator, which does not have the flange-shaped large-diameter portion, by supporting and fixing the insulator to the main metal fitting by a welded connection, an adhesive connection, shrink fitting or the like (see, for example, Patent Reference 2).    Patent Reference 1: JP-A 2002-164147    Patent Reference 2: JP-A 2002-158078